Wavelength-tunable semiconductor laser devices are widely employed in optical communication systems. The distributed Bragg reflector (DBR) laser is one type of wavelength-tunable laser that utilizes a periodic grating structure within the laser cavity to provide wavelength selectivity. Generally, the DBR has a gain section, which includes an active layer, and a grating section, which includes an active layer and a Bragg grating. The power of the output light from the DBR is controlled by current injection into the gain section, and the wavelength of the output light is controlled by current injection into the grating section.
One challenge for DBR laser design has been achieving a device with the stable, single longitudinal mode output desired for today""s moderate and long distance fiber optic communication applications. In a conventional DBR laser, the grating structure introduces a periodic modulation of the refractive index or gain within the laser cavity. This promotes the formation of two standing waves in the cavity (180xc2x0 out of phase with each other), often referred to as the degenerate Bragg modes. In order to achieve single-mode emission, the degeneracy must be xe2x80x9cbrokenxe2x80x9d so that only one of the Bragg modes will be pumped. Typically, the degeneracy is broken by reflections from the facets of the laser cavity, which promote laser oscillation of just a single Bragg mode.
As the DBR laser is wavelength-tuned, the phase relationship between the degenerate Bragg modes and the reflected facet light is altered. This change in phase relationship can cause the reflected light from the output facets to favor oscillation of a different Bragg mode, thus causing the laser output light to jump intermittently between Bragg modes during tuning. This xe2x80x9cmode hoppingxe2x80x9d results in an unstable and unpredictable device, and can make continuous wavelength tuning impossible.
The problem of xe2x80x9cmode hoppingxe2x80x9d in a DBR is typically eliminated by the inclusion of a third section of the laser device, specifically a phase section. FIG. 1 illustrates such a device, wherein the DBR contains a phase section 13 disposed between the gain section 12 and the grating section 11. Through proper biasing of the phase section 13 relative to the gain and grating sections 12, 11, the phase relationship between the degenerate Bragg modes and the facet reflections may be maintained in such a manner as to prevent mode hopping.
The three-section design necessitates complex coordination between the grating, gain, and phase sections of a DBR laser. For stable operation at desired power and wavelength conditions, microprocessors are typically used with factory-set memory conditions to carefully control the current to each of the three sections. Extensive testing and characterization are required for each device in order to determine the proper biasing conditions for each desired power and wavelength operation.
In general, the present invention relates to high-performance wavelength-tunable semiconductor laser devices for optical communication applications. Mode hopping can be advantageously avoided without the added expense and complexity of a phase section. Furthermore, complex control electronics are no longer required for balancing the current to three contacts in order to avoid mode hops, and overall device operation is greatly simplified.
Accordingly, an integrated semiconductor device comprises a wavelength-tunable laser on a substrate, where the laser has a gain section that includes an active layer, and a grating section that includes an active layer and a current-induced grating. A first electrical contact is provided over the gain section to supply current to the gain section and control the output power of the light, and a second electrical contact is provided over the grating section to supply current to the grating section and control the wavelength of the emitted light. According to one aspect, the wavelength-tunable laser is a distributed Bragg reflector (DBR) type laser.
The current-induced, or current injection grating of the present device causes gain in the active layer of the laser to be modulated spatially in the direction of light propagation, thus resulting in only one of the degenerate Bragg modes to oscillate. As the degeneracy of the Bragg modes is broken by current-injection, and not facet reflection, substantially continuous wavelength tuning is possible without the deleterious phenomenon of xe2x80x9cmode hopping.xe2x80x9d The present approach does not utilize a phase section, and thus eliminates the complex three-way interaction between the gain section, grating section, and a phase section.
The current-induced grating can be, for instance, a gain-coupled, or complex-coupled grating. In general, the grating strength is permitted to be strong relative to the facet reflections. In contrast to conventional index-coupled designs, grating structures having xcexaL products greater than 1.5, and typically between about 3 and 6, are employed, where xcexa (cmxe2x88x921) is a coupling coefficient relating to the extent that light is coupled backward over the distributed length of the laser cavity, and L (cm) is the length of the cavity.
In addition to the current-induced, or current-injection grating structures described above, the device may utilize alternative grating structures, including other gain-coupled, complex-coupled, and fractional wave shifted designs, with relatively strong coupling products (xcexaL greater than 1.5), which do not rely on facet reflections to break the degeneracy of the Bragg modes.
The integrated semiconductor device of the invention may include a directly modulated laser, or a laser coupled with an external modulator for forming the optical signals. For instance, the device may comprise a laser and an electroabsorption modulator (EML), or a Mach Zehnder modulator, integrated on a single substrate.
The invention further relates to methods for fabricating an integrated semiconductor device, comprising the steps of forming on a substrate a wavelength-tunable laser having a grating section adjacent to a gain section, the grating section having an active layer and a current-induced grating, and forming electrical contacts over each of the grating and gain sections.